Spectrum of Insecticide Cross-resistance in Pyrethroid-resistant Populations of Haematobia irritans (Diptera: Muscidae)

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Abstract
Pyrethroid (permethrin and fenvalerate)-resistant populations of Haematobia irritans (L.) were bioassayed with five pyrethroids: permethrin, fenvalerate, cypermethrin, flucythrinate, and deltamethrin. Resistance ratios (LD50 population ÷ LD50-susceptible population) for the Ben Hur population ranged from 6.9 for permethrin to 85.8 for cypermethrin, while those for the Red River population ranged from 12.1 to 35.3 for permethrin and cypermethrin, respectively. Resistance to DDT in the Red River population was 51.5-fold and there was only a low level of cross-resistance (0.8- to 4.8-fold) to a carbamate (bendicarb) and three organophosphorus (dioxathion, stirofos, sulprofos) insecticides. The level of synergism observed for cypermethrin in the resistant Red River population was 2.2- and 3.5-fold greater (for DEF and piperonyl butoxide, respectively) than that observed in the susceptible population, suggesting that metabolism alone cannot account for the level of resistance (35-fold) observed. Due to the broad spectrum of pyrethroid cross-resistance, the future of pyrethroids in ear tags for the control of H. irritans appears to be very limited.
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